The present invention relates to plasma torches and, in particular, is directed to a plasma torch circuit having an improved pilot arc to operating arc transfer circuit.
Plasma torches, also known as plasma arc torches, are well known in the art for their ability to cut and weld metal workpieces. Plasma torches operate by directing a plasma consisting of an ionized cloud of gas towards a workpiece. Such an example of a conventional single gas type plasma torch is disclosed in U.S. Pat. No. 3,813,510 to Hatch, and assigned to the assignee of the present invention, the patent of which is specifically incorporated herein by reference. Other patents disclosing plasma torches and specifically incorporated herein by reference are U.S. Pat. Nos. 4,225,769; 4,663,512; and 4,663,515.
As these patents illustrate, plasma torches operate by feeding an ionizable gas, such as nitrogen, through channels within the torch to the front end of the torch. The gas then may be caused to swirl in front of the end of a typically negatively charged electrode positioned at the front end of the torch. A tip that is positioned adjacent the end of the electrode and spaced a predetermined distance therefrom, has a sufficiently high voltage applied thereto to cause a spark to traverse the gap between the electrode and the tip to thereby complete an electrical circuit. This spark starts the pilot arc which is the flow of DC current.
The pilot arc consists of at least two parts. The first part is the actual current that flows from the electrode to the tip and which extends a distance from the tip and electrode by the movement of the swirling, ionized gas. The second part is a cloud of ionized gas that surrounds the actual current. The ionized gas cloud also extends from the tip and electrode, but at a distance further than the actual current. Typically, a DC current is supplied to the tip and electrode to maintain the pilot arc. The DC current may optionally be pulsed.
In order for the plasma torch to operate, a main or cutting arc must be established between the electrode and a workpiece. The process of switching the pilot arc from between the tip and the electrode into the main arc from between the electrode and the workpiece is known as transfer of the arc. In order to transfer the arc, a current has to flow between the electrode and the workpiece. Once this is accomplished the current between the electrode and the tip is generally stopped. A power circuit thus has one end that is coupled to the workpiece in order for the current to flow between the workpiece and the electrode. The height of the electrode and the tip to the workpiece is one of the factors effecting the ability of the plasma torch to achieve transfer. It is the cloud of ionized gas through which the transfer is initiated. Thus, current will conduct via the ionized cloud before the actual pilot current contacts the workpiece.
The pilot current is a low impedance path. On the other hand, prior to transfer the main or cutting current path is a higher impedance path because the ionized cloud acts like a gas tube (neon bulb) or a zener diode in that it takes a certain level of voltage across the distance between the electrode and the workpiece before conduction of current. The greater the distance between the electrode and the workpiece, the higher the impedance. However, once this breakdown voltage is reached, current flows and completes the path for transfer.
In the prior art, once transfer is achieved and current begins to flow in the workpiece, a current sensor open circuits the pilot arc current circuit. The current is then supplied between the electrode and the workpiece to maintain the established main arc. However, should the main arc not be established even though current has been sensed in the workpiece, it is necessary to reinitiate a pilot current. This redemand of current for the pilot arc and the extinguishment of the main arc current is hard on the torch and torch parts, increasing wear and tear, and reducing the overall life of the torch consumable parts.
Typical gas plasma torches include pilot arc circuits that provide a 5-40 amperage pilot arc current at 100-200 volts across the electrode to tip gap. In a typical plasma torch the typical components achieve an approximately 1/4" transfer distance. The greater the transfer distance the more suited the plasma torch is for automatic applications such as robotic welding and cutting. Furthermore, the greater the transfer height, the easier the plasma torch is to use, as the operator does not need to be as precise in positioning the torch adjacent the workpiece.
In traditional prior art plasma torch electrical circuits, there is a power supply for the pilot arc and a power supply for the main arc. Thus, in order to achieve a greater transfer distance, the amperage and voltage for the pilot circuit could be increased. However, increasing the voltage and amperage to a level where a greater transfer distance is achieved could create other problems.
Thus, there remains a need for an easy to use plasma arc torch that achieves a greater transfer height without utilizing typically necessary heavy, lossy circuit elements such as inductors, resistors or separate power sources.
It is an object of the present invention to provide a plasma arc transfer circuit that is proactive rather than reactive in establishing an operating arc in a plasma arc torch.
It is another object of the present invention to increase the transfer height in a plasma torch without significantly increasing the power requirement.
It is yet another object of the present invention to utilize a power circuit which includes a single power source that powers both the pilot arc and the main arc and that also increases the transfer height.
It is still another object of the present invention to provide a plasma arc transfer circuit that allows a plasma arc torch to adapt to robotic or mechanized plasma torch applications as well as everyday manual operations.